This invention relates to circuit breakers. More particularly, it is concerned with circuit breaker contact closing and opening mechanisms.
Circuit breakers typically employ a pair of separable contacts one fixed and one movable with the movable contact connected to an operating mechanism for opening and closing the contacts in response to manual operation of an operating handle. Circuit breakers also include a mechanism for automatically opening the contacts upon the occurrence of an overload condition. The handle is connected to the movable contact through a toggle arrangement, typically a spring operated overcenter linkage. The overload mechanism includes a sensing member such as a bimetallic and/or magnetic element which trips the toggle arrangement to produce rapid opening of the contacts upon sensing of an overload condition.
The toggle arrangement typically includes a large operating spring in order to provide rapid switching action. In certain breakers the operating spring has no direct effect on the force exerted by the movable contact against the fixed contact. A separate and smaller spring is used to generate the desired contact pressure. Upon opening of the contacts, however, either by tripping or by manual operation the contact spring tends to urge the movable contact toward the fixed contact. Thus, separation of the contacts is delayed, and the force on the contacts decreases continuously throughout the time of delay resulting in a period of inadequate contact force. Furthermore, the distance between the fully opened contacts is reduced by the amount of movement imparted by the contact spring.